My tomten has a zipper! I'm so delighted by this, I can hardly contain myself. It took two weekends to sew in - one to baste the zipper into place and the other to actually do the backstitch seams to secure the zipper to the sweater. But it's done! And, I didn't give in and use buttons when my initial installation attempts were less than successful.
I must have looked at five or six different zipper tutorials before diving in. First, I thought the stitch marker idea sounded best: you attach the fronts with stitch markers, then pin the zipper in place, then sew it. The stitch markers (i.e., paperclips, since that's what I use) didn't hold particularly well and pinning the zipper to garter stitch was an exercise in poking tiny holes in my fingers, not attaching a zipper. Then I realized that I really should baste the fronts together. It took a while but seemed to work swimmingly.
Then came time to attach the zipper to the sweater prior to sewing it in. Pins were clearly out at this point and I was very tempted by Brenda's suggestion to use a glue stick. But somehow I couldn't quite bring myself to touch glue to fabric. So I ended up whipstitching the edge of the zipper to the garment, eyeballing the placement and pushing here and there to make sure everything lined up just right. It was the perfect technique for this project. At that point, it was really just a matter of sewing the seams.
I certainly can't complain about the result! This Tomten gets the tadpole seal of approval - she wore it pretty much all day long. Even the hood (and that is saying something).